1. Field of the Invention
The present invention relates to an intake device of an internal combustion engine.
2. Description of the Related Art
In order to create a strong swirl motion in the combustion chamber when the engine is operating under a light load, and to obtain a high volumetric efficiency when the engine is operating under a heavy load, an engine is known in which the interior of the intake port is divided into a straight port and a helical-shaped port by a separating wall (Japanese Unexamined Patent Publication (Kokai) No. 58-48715). In this engine, a control valve is arranged in the straight port and this valve opens when the engine is operating under a heavy load. In addition, a bypass passage is branched from the straight port downstream of the control valve and is connected to the helix terminating portion of the helical portion of the helical-shaped port. In this engine, when the engine is operating under a light load, the control valve is closed. As a result, since the entire volume of air is introduced into the helical-shaped port, a strong swirl motion is created in the combustion chamber. Contrary to this, when the engine is operating under a heavy load, the control valve is open. As a result, the flow area of the intake port is increased, and a swirl motion created in the helical portion of the helical-shaped port is weakened by the air flowing into the helix terminating portion of the helical portion from the bypass passage. Thus, the volumetric efficiency is improved.
However, in this engine, since the inlet opening of the bypass passage is formed on the flat side wall of the separating wall, the amount of air flowing into the bypass passage is small when the control valve is open. As a result, it is difficult to sufficiently weaken the swirl motion created in the helical portion of the helical-shaped port, and thus a problem occurs in that a satisfactorily high volumetric efficiency cannot be obtained.